In order to increase the circuit density of a board including a base material having formed thereon a wiring pattern, or make it multifunctional or small-sized, there have been developed and proposed various boards such as one having interstitial via holes, and one including a base material on which a plurality of wiring patterns are laminated through an insulating layer.
Typical examples of conventional multilayer boards include the following. The term "double-sided board" as used herein refers to a board which has provided thereon a wiring pattern on each side thereof.
(1) Printed Circuits Handbook, chapters 33.2-33.8 (3rd Ed.; 1988) describes a multilayer board as shown in FIG. 2 in which double-sided boards 105 and 105' each having a wiring pattern 104 on each side thereof and a via hole 103 formed by plating a through hole, are laminated through a prepreg consisting of an adhesive resin, with connection between the wiring patterns of the laminated double-sided board being established by forming a common via hole 102 penetrating all the layers.
(2) Denshi Zairyo (Apr., 1991) page 103-108 describes a multilayer board as shown in FIG. 3. That is, a first layer wiring pattern 108 is formed on a base material 106. A second wiring pattern 109 made of a plating layer is formed on the first wiring pattern 108 through an insulating layer 107, and a third wiring pattern 109' made of a plating layer is formed on the second wiring pattern 109 through an insulating layer 107'. In addition, a fourth or more wiring patterns are formed, and a through hole 102 penetrating all the layers is formed. Finally, an electric source layer 110 is connected as an outermost layer.
(3) Japanese Utility Model Application Laid-Open No. 16482/1988 describes a board as shown in FIG. 4. That is, a base material 106 has provided thereon a first layer wiring pattern 108 made of a plating layer, and wiring patterns 109 and 109' are serially formed on the first layer wiring pattern 108 through insulating layers 107 and 107', respectively. The insulating layers between the wiring patterns are provided with interstitial via holes 111 filled with a conductive paste so as to be flush with the insulating layer to thereby establish electrical connection between the wiring patterns.
Among the aforementioned, the multilayer board described in (1) above is a popular one which is now put into practical use. In such a multilayer board, a plurality of double-sided boards 105 and 105' separately fabricated are laminated through a prepreg 101, which requires a very high precision in the alignment between the boards. That is, it is necessary that the through hole 102 provided after a plurality of boards are laminated so as to penetrate all the layers should certainly penetrate at predetermined places in each wiring pattern. If such alignment of the laminate is missed, an immediate fault occurs. Therefore, it has been necessary to ensure a penetration portion rather with some allowance in each wiring pattern. This hinders increase in the circuit density. When electroconduction is obtained between the through hole 102 penetrating all the layers and intermediate wiring patterns by means of the through hole 102, the contact area between the two is small and a problem arises in the reliability of electrical connection. In addition, through holes are provided also in those wiring patterns which do not need electroconduction, which limits freedom in wiring. This also hinders increase in the circuit density.
Further, the aforementioned multilayer board is very complicated to manufacture since it not only requires at least two plating operations in the step of manufacturing a unit board but also requires steps of laminating a unit board, producing an opening through the board, plating, etc.
The multilayer board described in (2) above also has a through hole 102 penetrating all the layers, and like the multilayer board described in (1) above, hinders increase in the circuit density. Wiring pattern is laminated on one surface of the base material, and hence the degree of increase in the number of electrical connection terminals between the wiring patterns is greater than the increase in the number of pattern layers. This also hinders increase in the circuit density.
Further, in the manufacture of the aforementioned multilayer boards, formation of an insulating layer and plating must be performed once each in order to form a single layer of wiring pattern. After all the wiring patterns have been formed, production of openings through the board and plating must be performed, and this leaves room for further improvement in the manufacture.
The multilayer board described in (3) above connects a plurality of wiring patterns laminated through an insulating layer by means of an opening provided in the insulating layer and filled with a conductive paste. However, the opening differs from a hole in that the opening has a closed bottom and hence the conductive paste filled and cured therein tends to form a space between it and the surface of the wiring pattern which it originally contacted since it shrinks upon curing. There is room for improvement in reliability of electrical connection. Similarly to (2) above, wiring patterns are laminated on one side of the base material, and the degree of increase of the number of electrical connection terminals is greater than the increase in the number of wiring pattern layers, which hinders increase in the wiring density.
Further, in the fabrication of the aforementioned multilayer boards, curing of the conductive paste filled in the insulating layer and the opening must be performed for each layer. Therefore, the board is subjected to thermal hysteresis twice for each connection between the wiring patterns, which not only makes the production procedure complicated but also causes a problem of thermal deterioration of the board.
In the multilayer boards described in (2) and (3) above, electronic components are mounted on only one surface of the board, leading to decrease in the mounting density.
Therefore, an object of the present invention is to solve the aforementioned problems encountered in the conventional techniques, and to provide a multilayer board which has a high reliability of electrical connection between wiring patterns laminated thereon, enables increase in circuit density, and also increase in the density of mounting components thereon.
Another object of the present invention is to provide a method of fabricating the aforementioned multilayer board.